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Piezoelectric Tap Switch

This circuit uses a flat piezoelectric wafer, glued inside a plastic box, as a finger tap sensor.  With each tap of a finger to the box holding the wafer, the circuit turns on and off AC or DC power to an external device.  The circuit is powered by a 9v battery and drives a single coil 5v latching relay with a 10 Amp contact rating.  The relay can switch AC or DC power.  In standby mode the circuit draws a low 1ua.

A firm finger tap to the plastic box cover causes the piezoelectric wafer to produce multiple voltage pulses.  The pulses are fed to a low power voltage comparator.  The comparator converts the low voltage pulses into 9v pulses.  Those pulses are fed to the clock input of one half of a CD4013 dual D flip/flop.  One section is configured as a half second one shot.  The second half is wired as a standard flip flop.  With this circuit, the flip/flop changes state with each finger tap to the box, holding the circuit.  The output of the flip/flop is connected to a dual FET push-pull driver circuit, which feeds power to a pair of capacitors.  The capacitors are wired in series with parallel diodes.  This forms a 235uF non-polarized capacitor.  With this charge pump configuration, the power pulses from the FETs feed either a positive pulse to the relay coil, to latch it, or a negative pulse to unlatch it.  The pulse lasts long enough to fully open or close the relay contacts.

Click on Drawing Below to view PDF version of Schematic

Piezeoelectric Tap Relay circuit, designed by Dave Johnson, PE

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